A formal investigation is under way following a chemical leak at Dounreay on Wednesday night.
Emergency crews were called to the prototype fast reactor of the former nuclear facility at around 8pm after reports of a leak from the sodium storage tanks in the building.
The leak is believed to have been caused by a planned reaction of material inside one of the tanks.
This resulted in the release of a small amount of process chemicals and tritium, which presents a low radiological hazard.
The Scottish Environment Protection Agency (Sepa) were notified of the incident shortly after midnight on Thursday, with specialist officers stationed at the scene overnight.
Dounreay, near Thurso, was the UK site for the development of fast reactor research from 1955 to 1994.
The facility on the north Caithness coast – including the distinctive dome-shaped structure – is now in the process of being closed down, demolished and cleaned up.
A spokesman for Dounreay Site Restoration Ltd (DSRL), which is currently in charge of the decommissioning of the site, said: “At approximately 8pm on April 20, site emergency personnel responded to a call at the sodium storage tanks at the Prototype Fast Reactor complex.
“A planned reaction of material inside a tank resulted in an excursion, causing the release of a small amount of caustic liquor.
“No one was injured and damage was confined to components in the tank. We believe the environmental impact is low. Emergency personnel kept the tank under observation as a precaution.
“Sepa and ONR have been informed of the occurrence, and DSRL today is commencing its own investigation of the circumstances.”
‘Risk to the public and the environment extremely low’
While the leak is considered to pose “extremely low” risk to the public and the environment, Sepa has now launched a formal investigation into the incident.
Radioactive substances manager at Sepa, Paul Dale, said: “Shortly after midnight on Thursday, Sepa was notified of an incident at the decommissioning Dounreay nuclear site at Thurso in the Scottish Highlands.
“Specialist Sepa officers were in liaison with the site operators overnight and understand that the incident occurred at a sodium storage facility associated with the Prototype Fast Reactor (PFR) building, currently being decommissioned.
“Further updates have been received today and we understand that there was a small release of process chemicals from the system and a release of tritium, which presents a low radiological hazard, may have occurred.
“Whilst from initial information we consider the risk to the public or the environment to be extremely low, Sepa has launched a formal investigation and will provide further updates as appropriate ”
A spokesman for ONR added: “We have been made aware of an incident at the Dounreay site yesterday evening, April 20, that resulted in site emergency personnel attending to a call at the sodium storage tanks at the Prototype Fast Reactor complex.
“ONR is content that Dounreay responded appropriately and in accordance with their established arrangements and the system is now stable.
“Our inspectors are in close contact with Dounreay and will continue to monitor the situation, including Dounreay’s investigation into the incident.”